Extensive researches have been made heretofore on catalyst systems for uniformly dimerizing lower monoolefins such as ethylene, propylene and butene. As the catalysts, Ziegler catalysts having transition metals as central catalyst components are usually advantageous in terms of selectivity to dimers of lower monoolefins. In particular, when a catalyst obtained from a mixture of a nickel compound and an organoaluminum halide compound is used, good results have been obtained with respect to both dimerization activity and selectivity. Further, with respect to dimerization of butenes, a catalyst system consisting of a nickel compound selected from the group consisting of a nickel salt of a higher mono- or di-carboxylic acid having from 5 to 20 carbon atoms and a coordination complex of an organic phosphine and a nickel halide, an organoaluminum compound and hydrogen, has been proposed as a means for providing an improved yield of octenes, in Japanese Examined Patent Publication No. 42249/1991.
It is generally known that in these dimerization reactions of butenes, the resulting octenes are obtained not only as a mixture of various skeletal isomers such as n-octene, 3-methylheptene and 3,4-dimethylhexene but also as a mixture of various double-bond isomers as a result of isomerization reactions which occur competitively with the dimerization reactions. Further, it is already known that alcohols having 9 carbon atoms (hereinafter referred to as "INA") obtained by subjecting the octenes obtained in the above dimerization reaction to a hydroformylation reaction and a hydrogenation reaction are advantageously used as a raw material of a plasticizer for a vinyl chloride resin (U.S. Pat. No. 789,777 and Japanese Examined Patent Publication No. 15849/1986). Further it is known that INA is excellent alcohols for a plasticizer for a vinyl chloride resin in respect of heat resistance and cold temperature flexibility as compared with 2-ethylhexanol which is obtained by hydroformylation, dimerization and hydrogenation of propylene and which has been used commonly and extensively as an alcohol for a raw material of a plasticizer.
On the other hand, as a method for producing octenes other than dimerization of butenes, there is an oligomerization reaction of olefins. For example, Japanese Unexamined Publication No. 132534/1989 discloses a method for oligomerizing olefins having from 2 to 10 carbon atoms by means of a zeolite catalyst surface-treated with a trialkylpyridine compound or an organic phosphorus compound.
As described above, the octenes obtained by dimerization of butenes have linear and various branched structures and are obtained as a mixture of many isomers having double bonds at different positions.
When these products are used, for example, as starting materials of the hydroformylation reaction described above, octenes having high degrees of branching will remain in the system as unreacted substances since in general, octenes having lower degrees of branching are more reactive in the hydroformylation reaction. Further, when INA produced by hydroformylation and hydrogenation reactions of octenes is used as a raw material of a plasticizer, INA having a low degree of branching has an advantage of providing a product excellent in important properties of a vinyl chloride resin such as cold resistance and resistance to volatilization.
However, in a dimerization reaction of butenes, a reaction substrate usually contains a large amount of internal olefins such as 2-butene and it is quite difficult to obtain octenes having low degrees of branching economically in a high yield from such a reaction substrate that contains a large amount of internal olefins. With respect to the above-mentioned catalyst system for dimerizing n-butenes proposed in Japanese Examined Publication No. 42249/1991, the resulting octene mixture is not very suitable for industrial use, in particular, for use in the field of plasticizer as described above, because of the high production ratio of 3,4-dimethylhexene having a high degree of branching. The above-mentioned method for producing INA proposed in U.S. Pat. No. 789,777 and Japanese Examined Patent Publication No. 15849/1986 is not necessarily satisfactory in this respect.
Further, the method for oligomerizing olefins proposed in Japanese Unexamined Patent Publication No. 132534/1989 is not necessarily satisfactory as a method for producing octenes economically and selectively, when it is taken into consideration that the fractional distillation process of the product obtained as a mixture of olefins having a wide range of number of carbon atoms is complex and that the selectivity to octenes in the product is not necessarily sufficient.
Accordingly, objects of the present invention are to develop a catalyst system capable of improving the yield of octenes and the selectivity to octenes having low degrees of branching in dimerization of butenes and to develop a method by which INA having a low degree of branching can be produced selectively in a high yield.